[Cronos]

They are citizens of the United Kingdom of Great Britain and Northern Ireland, AND they are subjects of the Queen

[Cronos]

Boris will deliver

[Winniesboy]

The term 'subject' lost its last vestiges of constitutional or legal validity nearly 100 years ago de jure, and long before that de facto. The only circumstances in which you will come across it these days are in the archaisms of formal state occasions such as the pantomime State opening of Parliament, which are retained primarily for the entertainment of tourists (especially American). In all current constitutional and legal usage, it has long been supplanted by 'citizen'.